As a fuel for an internal combustion engine, a composite fuel is known, in which alcohol such as ethanol and gasoline are mixed. A fuel containing alcohol can be used in internal combustion engines for gasoline. However, alcohol and gasoline have different properties, and therefore, when the fuel containing alcohol is used in an internal combustion engine, control of the internal combustion engine with consideration given to the difference in property is required. For example, Patent Documents such as Japanese Patent Laid-Open No. 2006-283636, Japanese Patent Laid-Open No. 5-1574, Japanese Patent Laid-Open No. 2003-286848, Japanese Patent Laid-Open No. 2005-83216, Japanese Patent Laid-Open No. 2008-180103, and Japanese Patent Laid-Open No. 2007-198308 disclose the inventions relating to control of internal combustion engines when using fuels containing alcohol.
What is remarkable as the difference in property between alcohol and gasoline is the distillation characteristics. The graph of FIG. 6 shows the distillation characteristic of a high-concentration ethanol composite fuel (E85) and the distillation characteristic of gasoline (E0) by comparison. The axis of abscissas of the graph represents a distillation ratio, whereas the axis of ordinates represents a distillation temperature. As shown in the graph, the fuel containing alcohol has a low evaporativity in a low temperature region as compared with gasoline. This is because alcohol has fewer low-boiling point components as compared with gasoline.
The low volatility of alcohol becomes a problem especially at the time of cold start of an internal combustion engine. At the time of cold start of an internal combustion engine, fuel atomization becomes worse as compared with the time after completion of warming up, and therefore, unburned HC which does not contribute to combustion easily adheres to the wall surface of the cylinder. The unburned HC adhering to the cylinder wall surface is scraped up by the ascent of a piston, and is released to an exhaust passage by being contained in the combustion gas when an exhaust valve is opened. Therefore, the combustion gas which is discharged from the internal combustion engine at the time of cold start contains a large amount of unburned HC as compared with the time of completion of warming up. When the fuel contains alcohol, especially in the case of a high concentration alcohol mixed fuel, the amount of the fuel adhering to the cylinder wall surface increases with low volatility of the alcohol mixed fuel, and as a result, the amount of unburned BC discharged from the internal combustion engine also increases.
A catalyst for removing harmful components in the combustion gas is disposed in the exhaust passage of an internal combustion engine. The catalyst is also effective for the fuel containing alcohol, but the catalyst cannot sufficiently exhibit the purification ability in a low-temperature state. Therefore, in the state in which the catalyst temperature is low and the catalyst is inactive as the state at the time of cold start of an internal combustion engine, the unburned HC contained in the combustion gas is not sufficiently purified, and is released into atmosphere. In order to prevent worsening of the exhaust emission at the time of cold start, reduction in the exhaust amount of unburned HC itself is also required in the internal combustion engine using a fuel containing alcohol.